Augmented epithelial multidrug resistance-associated protein 4 expression in peritoneal endometriosis: Regulation by lipoxin A₄

Objective: To compare the expression of the prostaglandin (PG) E ₂ transporter multidrug resistance-associated protein 4 (MRP4) in eutopic and ectopic endometrial tissue from endometriosis patients with that of control subjects and to examine whether MRP4 is regulated by the antiinflammatory lipid lipoxin A₄ (LXA₄) in endometriotic epithelial cells. Design: Molecular analysis in human samples and a cell line. Setting: Two university hospitals and a private clinic. Patient(s): A total of 59 endometriosis patients and 32 age- and body mass index-matched control subjects undergoing laparoscopy or hysterectomy. Intervention(s): Normal, eutopic, and ectopic endometrial biopsies as well as peritoneal fluid were obtained during surgery performed during the proliferative phase of the menstrual cycle. 12Z endometriotic epithelial cells were used for in vitro mechanistic studies. Main Outcome Measure(s): Tissue MRP4 mRNA levels were quantified by quantitative reverse-transcription polymerase chain reaction (qRT-PCR), and localization was analyzed with the use of immunohistochemistry. Cellular MRP4 mRNA and protein were quantified by qRT-PCR and Western blot, respectively. PGE₂ was measured in peritoneal fluid and cell supernatants using an enzyme immunoassay (EIA). Result(s): MRP4 was expressed in eutopic and ectopic endometrium, where it was overexpressed in peritoneal lesions and localized in the cytoplasm of glandular epithelial cells. LXA ₄ attenuated MRP4 mRNA and protein levels in endometriotic epithelial cells in a dose-dependent manner, while not affecting the expression of enzymes involved in PGE₂ metabolism. Investigations employing receptor antagonists and small interfering RNA revealed that this occurred through estrogen receptor α. Accordingly, LXA₄ treatment inhibited extracellular PGE₂ release. Conclusion(s): We report for the first time that MRP4 is expressed in human endometrium, elevated in peritoneal endometriosis, and modulated by LXA₄ in endometriotic epithelial cells.